| GrowthBook Homepage | https://growthbook.io |
We built this because our community of 5,000+ engineers kept asking for better ways to prove impact. One customer told us: "Holdouts helped us discover our 'successful' feature was actually causing a 2% revenue decline after 30 days."
The best part? It's all open source, warehouse-native, and transparent - you can audit exactly how we calculate everything.
Want to see the implementation details? Check out our deep dive: https://blog.growthbook.io/holdouts-deep-dive/
Think of them as your experiment's control group that persists even after launch. While 95% of users get your shiny new feature, 5% stay in the original experience. This gives you a clean baseline to measure against.
🪝 Catch delayed effects - Some changes take weeks to show impact
⚖️ Measure cumulative impact - Stack multiple features and still understand individual contribution
🤝 Build trust in your data - Show stakeholders exactly what drove that revenue increase
Ever wonder if your feature actually moved the needle, or if that metric bump was just seasonal variance?
Most teams struggle with this because they can't isolate the real impact of their changes.
That's where holdouts come in - and why we just shipped them in GrowthBook.
What are holdouts?
Yesterday's comic was, of course, in reference to Twyman's Law.
Applicable not only to experiment analysis, but also life. If it looks too good to be true, then it probably is 😅
Here's our full guide: https://blog.growthbook.io/guide-to-geo/
Question: Is anyone A/B testing GEO yet? How is it working?
• There's no page 2 in AI search - you're either in the answer or you're invisible
• Statistics boost AI visibility by 33.9% - AI can't create data, so it gravitates toward sources that provide it
• Most AI traffic is invisible - it shows up as "Direct" in analytics because AI doesn't pass referral data
The kicker? ChatGPT already drives 10% of new signups for companies like Vercel. This isn't future speculation - it's happening now.
As a company, we do things companies do:
• Run A/B tests (obviously)
• SEO
• And now, GEO - optimizing for visibility in AI searches like ChatGPT
What's GEO (Generative Engine Optimization)? We were wondering that too, so we did some research.
Here are 3 surprising findings:
This week we've been sharing everything included in our latest 4.1 release, which includes:
→ 150+ commits
→ 8 major features (holdouts, dashboards, AI, Vercel Native, Pre-computed dimensions, MCP update, FerretDB, Sanity)
→ 0 vendor lock-in
Curious?
🔗 Full release notes: https://blog.growthbook.io/growthbook-version-4-1/
We're just going to go ahead and say it: GrowthBook is the most comprehensive open-source experimentation platform.
Now with #FerretDB support.
Because open source means ALL the way down.
Learn more: https://docs.growthbook.io/self-host/ferretdb
